The growth in demand for telecommunication services is increasing at an ever-quickening pace. The majority of the demand is being driven by the explosion in the use of the Internet and a steady stream of new applications being introduced which further increase the demand for increased bandwidth. Currently, a large portion of the Internet traffic is still carried by circuit switched transport facilities. In the case of Metropolitan Area Networks (MANs), most of the traffic is transported over SONET/SDH based networks most of which were originally resigned for voice traffic. With time, more and more customers are using the networks for transporting data rather than voice.
The requirements for networked communications within the user community have changed dramatically over the past two decades. Several notable trends in the user community include (1) the overwhelming domination of Ethernet as the core networking media around the world; (2) the steady shift towards data-oriented communications and applications; and (3) the rapid growth of mixed-media applications. Such applications include everything from integrated voice/data/video communications to the now commonplace exchanges of MP3 music files and also existing voice communications which have begun to migrate towards IP/packet-oriented transport.
Ethernet has become the de facto standard for data-oriented networking within the user community. This is true not only within the corporate market, but many other market segments as well. In the corporate market, Ethernet has long dominated at all levels, especially with the advent of high-performance Ethernet switching. This includes workgroup, departmental, server and backbone/campus networks. Even though many of the Internet Service Providers (ISPs) in the market today still base their WAN-side communications on legacy circuit oriented connections (i.e. supporting Frame Relay, xDSL, ATM, SONET), their back-office communications are almost exclusively Ethernet. In the residential market, most individual users are deploying 10 or 100 Mbps Ethernet within their homes to connect PCs to printers and to other PCs (in fact, most PCs today ship with internal Ethernet cards) even though the residential community still utilizes a wide range of relatively low-speed, circuit-oriented network access technologies.
The use of Ethernet, both optical and electrical based, is increasing in carrier networks due to advantages of Ethernet and particular Optical Ethernet, namely its ability to scale from low speeds to very high rates and its commodity-oriented nature. With the rapid increase in the demand for user bandwidth, and the equally impressive increase in the performance of Ethernet with the LAN environment, the demand for Metropolitan network performance is rapidly increasing. In response, there has been a massive explosion in the amount of fiber being installed into both new and existing facilities. This is true for both the corporate and residential markets.
A problem arises, however, how to transfer legacy TDM traffic over an asynchronous Ethernet network and particularly, how to extract and reconstruct the TDM clock from the received data at the other side. It is important that the clock used at the receive side be traceable to the clock used at the transmitter. Better still, it is desirable that the clocks on either end of a connection be traceable to the centralized clock source and not just to each other. The clock at the transmitter side can be provided from an external source, a clock distribution network or from SONET/SDH equipment.
Circuit Emulation Service (CES) modules are used in Ethernet networks to transport TDM streams over asynchronous networks. In order to transmit TDM traffic, CES modules require some form of clocking. Typically several clocking options include: (1) an external clock input from, for example, a E1 or T1 connection in accordance with specification G.703; (2) line clock which is recovered from a E1/T1 or STM port (i.e. loop timing); and (3) a clock derived from an internal free running local oscillator.
From the standpoint of a service provider, in addition to basic connectivity, one of the services typically provided is an accurate clock source for customers to use. The clock source provided should be of high enough quality that customers can utilize the clock for transmission of the data from their customer equipment to a remote destination. A clock derived from the local oscillator in a CES module typically does not have sufficient accuracy to meet the necessary requirements.
Considering a connection between two CES modules, on the CES module on one side of the connection is configured as a master and placed in free running mode while the CES module on the other end of the connection synchronizes to it. The accuracy of the clock on both ends of the connection, however, is limited to the accuracy of the local oscillator in the CES module acting as the master.
The quality of the clock source used by the CES modules in the network can be greatly increased by use of a highly accurate centralized clock. Thus, there is a need for a mechanism of distributing a high accuracy clock to a plurality of CES modules. The mechanism should be able to distribute the clock to multiple CES modules over an asynchronous Ethernet network.